opamp design at 4V supply with gate oxide break down 1V

[viperpaki007]

Hi,

I am designing a simple operational amplifier, which should work at 4V supply and have an output voltage swing from 3.7 to 0.5 V. I made the following design. The design is working fine at supply voltage of 1.2 volts. Now i have to make it working for supply voltage to 4V. Considering the breakdown voltage of each transistor is 1V. How can i increase the supply voltage? Do i have to use cascode transistors now? if yes then in which way?

 

[Dominik Przyborowski]

Yes, You cannot exceed 1V on Vgd and Vgs voltages on any transistor, so to use cascodes and some diodes especially at output stage.

 

[viperpaki007]

correct me if i am wrong, M7, M9 and all the transistors in output stages need to be cascoded

 

[Dominik Przyborowski]

In ouput source follower You need to add diode connected transistors to avoid vgd and vgs exceed 1V for large signals. Each current source/sink should be cascoded.
Check this paper: https://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5642503

BTW. I assume You are using n-well process so nmos bulks should be connected to ground unless You are using triple-well nfets. And of course M10 should be diode connected ;-)

 

[viperpaki007]

Thank you for the help.. I have modified the opamp structure (diagram attached) using cascoded transistors in first stage of opamp. However, the 2nd stage consist of CS amplifier. How can i maintain the gate oxide breakdown limitations for CS M10 transistor. Will cascoding over M10 not cause the circuit response to change?.. By the way, now i am using 2.5V gate oxide devices because with 1V devices, the number of cascoded stages were quite many.